1. Field of the Invention
The present invention relates to a radiation imaging system capable of capturing a radiation image of an object using a radiation ray that passes through the object, a control method of the system, and a program used for causing a computer to execute the control method.
2. Description of the Related Art
An X-ray imaging apparatus is used for capturing an image using X-ray which is one type of radiation rays. In recent years, a digital X-ray imaging apparatus using a photoelectric conversion element has been used in acquiring intensity distribution of an X-ray beam that passed through an object. The digital X-ray imaging apparatus is considered advantageous over a conventional film-type imaging apparatus in that it can produce images with high sensitivity and better image quality. Further, since the images captured by the digital X-ray imaging apparatus are stored in digital data, various types of image processing can be performed after the imaging, and the captured images can be processed into images that can be easily diagnosed. Further, the digital X-ray imaging apparatus is advantageous to management of the captured images and transfer of the image data via a network.
Generally, when X-ray imaging is performed, a space where the imaging is actually performed by irradiating X-ray is different from a space where the X-ray imaging apparatus is controlled and an obtained image is displayed for diagnosis. Further, a room where the imaging is performed and a room where the control is performed can be different depending on an imaging style. This is because there is a need for separately using an “X-ray sensor unit” which is used for actually taking the X-ray image, and a “main control unit” which controls the apparatus and performs image processing.
In an X-ray imaging system including such an X-ray sensor unit and a main control unit, the X-ray sensor unit includes an X-ray sensor portion for acquiring an X-ray image of an object and also a communication interface (I/F) for transmitting the acquired image data to a distant main control unit. The X-ray sensor unit performs an imaging operation according to an instruction given from the main control unit. On the other hand, the main control unit includes an image processing unit which converts the image data transmitted from the X-ray sensor unit into image data appropriate for a diagnosis, a control unit which controls the X-ray imaging system, and a user I/F.
The X-ray sensor portion in the X-ray sensor unit includes a two-dimensional array of conversion elements and switching elements such as a thin film transistor (TFT). In obtaining an X-ray image of an object, the object is positioned between an X-ray source and the X-ray sensor unit. Then, an amount of X-ray that passed through the object is converted into an electric signal by each conversion element. In this way, the X-ray image is obtained. Further, the electric signal (X-ray image signal) output from each conversion element is individually read out and digitized according to analog-to-digital (A/D) conversion.
In recent years, an X-ray sensor portion which can capture moving images as well as still images has been developed. Such units are discussed, for example, in Japanese Patent Application Laid-Open No. 10-285466 and Japanese Patent Application Laid-Open No. 2006-43293. From the viewpoint of work efficiency and space savings, there is a growing demand for an X-ray imaging apparatus which can capture moving images and still images.
The X-ray image obtained by the X-ray sensor portion includes image reception characteristics (sensor characteristics), such as variation in a stationary noise and sensitivity characteristics. Since the sensor characteristics are unique to the X-ray sensor portion, they need to be corrected. For example, the X-ray sensor unit corrects the sensor characteristics and outputs an X-ray image having the sensor characteristics corrected to the main control unit of a subsequent stage. The main control unit processes the X-ray image transmitted from the X-ray sensor unit so that the X-ray image can be diagnosed more easily. The processing performed by the main control unit includes, for example, sharpening the image, reducing granularity, and converting gradation. After the image processing, the X-ray image is displayed on a monitor or stored in a memory.
If the configuration of the X-ray sensor unit is such that the above described correction processing of the sensor characteristics and the image processing, such as sharpening, granularity reduction, and gradation conversion are performed internally, it will increase a cost, heat generation, and size of the X-ray sensor unit.
On the other hand, if the configuration of the X-ray imaging system is such that the X-ray sensor unit outputs a captured X-ray image to the main control unit without processing it, and the main control unit performs the correction processing of the sensor characteristics and all the image processing, the cost, heat generation, and size of the X-ray sensor unit will not be increased. However, in this case, since the main control unit needs to perform the correction processing of the sensor characteristics, the main control unit needs to comprehend all of the image reception characteristics unique to the sensor, and a problem of inconsistency between the X-ray sensor unit and the main control unit may occur.
For example, if the X-ray sensor unit is replaced with another one, the main control unit needs to acquire the sensor characteristics of the new X-ray sensor unit in some way. Further, since the correction processing of the sensor characteristics is performed by the main control unit, offset data necessary in correcting the sensor characteristics needs to be transferred from the X-ray sensor unit at the time the image is captured. This causes increase in communication traffic volume. Since the X-ray sensor unit and the main control unit are often located some distance apart, the increase in the communication traffic volume will be a major problem.
Regarding the image processing performed by the main control unit of the subsequent stage, in some cases, a parameter used for image adjustment changes according to a type of the captured image (e.g., imaging portion). In order to automatically adjust an image quality to a level appropriate for the diagnosis, it is necessary to analyze the captured image and determine the parameter.
FIGS. 8A and 8B are schematic diagrams illustrating an example of a gradation conversion curve used in general gradation conversion processing. In FIGS. 8A and 8B, a mean pixel value (also called a pixel mean value) of an image is calculated as a gradation conversion parameter (gradation conversion curve) illustrated in FIG. 8A. Then, the gradation conversion curve illustrated in FIG. 8A is shifted so that an input reference value (focus of attention) illustrated in FIG. 8B is maintained at a predetermined luminance value (output designated value) on a display monitor. The pixel value of the captured image is shifted (performed gradation conversion) according to the shifted gradation conversion curve. A method for such gradation conversion processing is discussed, for example, in Japanese Patent Application Laid-Open No. 2001-325594.
On the other hand, if the correction processing of the sensor characteristics is performed by the X-ray sensor unit, transmission of the data which is used in correcting the image to the main control unit will be unnecessary. This is advantageous from the viewpoint of a communication band. Further, the main control unit does not need to acquire the characteristics unique to the X-ray sensor unit, and the consistency between the X-ray sensor unit and the main control unit can be improved. Furthermore, since a load of the main control unit can be reduced, causing the X-ray sensor unit to perform the correction processing is also effective from the viewpoint of load sharing.
As described above, the configuration of the X-ray imaging system where the X-ray sensor unit transmits the image data to the main control unit after performing correction processing of the sensor characteristics, and where the main control unit performs image processing so that the image is appropriate for diagnosis brings about significant advantages.
However, according to the above described configuration where the X-ray sensor unit performs correction processing of the sensor characteristics and outputs the corrected image data to the main control unit, the X-ray sensor unit needs to temporally store pixel value data for one image therein for performing the correction processing of the sensor characteristics. In this case, since the image data is transmitted to the main control unit after the correction processing of the sensor characteristics is performed on the stored image data, delay may cause in the transmission of the image data to the main control unit. Further, since the main control unit also needs pixel value data of one image in calculating the pixel mean value of the image data, the main control unit needs to receive the image data for one image from the X-ray sensor unit and temporally store it.
FIG. 9 is a timing chart of a conventional example of a series of operations in the X-ray imaging, from X-ray irradiation to an object to display of the X-ray image.
According to the example illustrated in FIG. 9, after the main control unit receives all pixel data of one image, it calculates the pixel mean value as a gradation conversion parameter according to image analysis. Then, the image data which has been subjected to the image processing using the gradation conversion parameter is transmitted to the display monitor to be output. Thus, the time necessary in transferring the image data from the X-ray sensor unit to the main control unit may be a bottleneck, and delay time that occurs in the display of the image may be considered as a major inconvenience to the user.
The X-ray sensor unit developed in recent years can process images with finer definition. A pixel matrix of a general digital X-ray sensor unit is a few thousands×a few thousands (e.g., 2000×2000) pixels. Data for one pixel is about 8 to 16 bits. Thus, it is necessary to transfer large quantities of data. Further, in capturing a moving image, a real time image display is desired. Thus, if display of the image is considerably delayed, a difference between the operation and visual recognition will increase and will cause additional inconveniences.
Under such circumstances, in reducing the delay in the image display, it is necessary to improve a data transfer rate by increasing the number of bits transferred in a data transfer path or increasing a speed of data transfer. However, in increasing the bit numbers, a large diameter cable will be necessary. If such a cable is used, portability will be decreased. On the other hand, in increasing the data transfer speed, costs of components used in an input/output (I/O) unit will be increased and, further, securing a transmission quality will be difficult.
Further, as a method for preventing occurrence of the delay in the display, the main control unit can give priority to the display of the image. In such a case, after receiving the image data from the X-ray sensor unit, the main control unit may perform the image analysis such as calculation of the pixel mean value in parallel with the image processing such as gradation conversion.
FIG. 10 is a timing chart of a conventional example of a series of operations in the X-ray imaging when the display of image is prioritized, from X-ray irradiation to an object to display of the X-ray image.
According to the example illustrated in FIG. 10, image processing necessary in displaying the image is performed during the image analysis, so that the display of the image is prioritized and the delay in the display is prevented or at least minimized. In this case, a parameter obtained from the image analysis is reflected to the image processing of a next frame or later. For example, if an “i” frame is necessary in calculating an analysis parameter, the image processing of an “N”-th frame uses an analysis parameter of an image of an “N−i”-th frame. Similarly, the analysis parameter of the image of the “N”-th frame is reflected to the image processing of an “N+i”-th frame.
However, according to this method, the parameter obtained from an analysis result such as the pixel mean value cannot be reflected to the image processing in the frame at that time. In other words, the parameter is reflected after one frame or more. Thus, it is difficult to accurately process the image according to the analysis result.
Further, in imaging of an X-ray moving image, an irradiation amount of the X-ray is automatically adjusted at real time so that an optimum image for diagnosis is provided. According to this X-ray control, for example, if luminance of the whole X-ray image is low, the irradiation amount of the X-ray is increased, and if the luminance of the whole X-ray image is high, the irradiation amount of the X-ray is decreased. In this way, feedback of the X-ray control parameter which is calculated according to the image analysis is performed, and the irradiation of the X-ray is controlled according to the value. In this case, the pixel mean value, for example, is used as the X-ray control parameter.
Thus, in calculating the X-ray control parameter, the main control unit performs image analysis such as the calculation of the pixel mean value after receiving all the pixel data of one image, and then transmits the calculated X-ray control parameter to the X-ray generating apparatus. Thus, the time necessary in the image data transfer becomes a bottleneck, and the delay in the X-ray control may be increased or the reflection of the X-ray control parameter may be delayed a few frames.